The broad objectives of this multi-faceted research plan are: 1) to improve the use of attenuated Salmonella typhi as a "live vector vaccine" that can deliver foreign antigens to the human immune system and elicit protective responses, particularly serum and mucosal antibodies; 2) to construct an S. typhi-based, single-dose oral vaccine to protect against diphtheria, tetanus and pertussis, as an initial step towards meeting the challenge of the Children's Vaccine Initiative. To achieve these broad objectives, we will first modify attenuated S. typhi strain CVD 908 (already shown in clinical trials to be a well-tolerated and highly immunogenic live oral typhoid vaccine) to enhance even further its properties as a live vector. We will next optimize in vivo expression of foreign antigens by modifying the timing and level of gene expression. This will be accomplished by identifying preferred promoters and by determining the importance of gene copy number. The basic hypotheses embodied in the various constructs that we propose will be tested via clinical studies in adult volunteers (funded by other sources). We will apply this information as we develop an S. typhi-based oral vaccine expressing diphtheria, tetanus and pertussis antigens.